A ruby is a pink to blood-red gemstone, a variety of the mineral corundum (aluminium oxide). The red color is caused mainly by the presence of the element chromium. Its name comes from , Latin for red. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones, together with the sapphire, the emerald, and the diamond.
Prices of rubies are primarily determined by color. The brightest and most valuable "red" called pigeon blood-red, commands a huge premium over other rubies of similar quality. The primary hue of any ruby must be red. All other hues of the gem species corundum are called sapphire. Ruby may exhibit a range of secondary hues. Orange, purple, violet and pink are possible.Â After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions may indicate that the stone has been treated.Â Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.0.
All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as "silk". Gemologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes. Usually the rough stone is heated before cutting. Almost all rubies today are treated in some form, with heat treatment being the most common practice.Â Â Heat is usedÂ to improve color, remove , blue patches and silk. These heat treatments typically occur around temperatures of 1800 Â°C (3300 Â°F).The silk is only partially broken as the color is improved.
The Mogok Valley in Upper Myanmar (Burma) was for centuries the world's main source for rubies. That region has produced some of the finest rubies ever mined, but in recent years very few good rubies have been found there.
Rubies have historically been mined in Thailand, Cambodia, Afghanistan, Tanzania, Madagascar, Vietnam, Nepal, Tajikistan, and Pakistan. More recently, large ruby deposits have been found under the receding ice shelf of Greenland.Â Sri Lanka has deposits of pink sapphires.Â Â Spinel, another red gemstone, is sometimes found along with rubies in the same gem gravel or marble.
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